On the eigenvalues and Seidel eigenvalues of chain graphs

In this paper we consider the eigenvalues and the Seidel eigenvalues of a chain graph. An$\text{dbar}$elić, da Fonseca, Simić, and Du \cite{andelic2020tridiagonal} conjectured that there do not exist non-isomorphic cospectral chain graphs with respect to the adjacency spectrum. Here we disprove this conjecture. Furthermore, by considering the relation between the Seidel matrix and the adjacency matrix of a graph, we solve two problems on the number of distinct Seidel eigenvalues of a chain graph, which was posed by Mandal, Mehatari, and Das \cite{mandal2022spectrum}.